1. Field of the Invention
The present invention relates to antennas, and more particularly, to a dual band antenna for mobile communications.
2. Description of the Related Art
With the rapid progress of mobile communications, the capacity of existing systems is becoming saturated, and thus, new systems are being developed at new frequencies to enhance capacity. Accordingly, the interrelationship between existing and new systems must be taken into consideration in the design of mobile communications equipment. For mobile communications antennas, major design concerns are power efficiency and effective use of frequency.
In practice, it is desirable in the Republic of Korea (South Korea) to interlink the existing CDMA (Code Division Multiple Access) system with the new PCS (Personal Communication System) system, in the U.S.A. to interlink the existing AMPS (Advanced Mobile Phone Service) system with the PCS system, and in Europe to interlink the existing GSM (Groupe Speciale Mobile) system with the DCS (Digital Communication System) 1800 system. Generally, a "dual band system" is a system that allows for communications within two different systems at different frequency bands, such as in above examples. It is desirable to manufacture communications equipment capable of operating within dual band systems.
Heretofore, each radio telephone terminal in the dual band systems are provided with two separate miniature antennas for two different bands, which results in increased production cost. Also, the use of two antennas for this purpose is an obstacle to the miniaturization of the radio telephone terminal, and results in an inconvenience to the user. For these reasons, it is required to develop a dual band antenna capable of being used for both bands.
U.S. Pat. No. 4,509,056 discloses a multi-frequency antenna employing a tuned sleeve choke. Referring to FIG. 1, an antenna of the type disclosed in that patent is shown. This antenna operates effectively in a system in which the frequency ratio between operating frequencies is 1.25 or higher. The internal conductor 10 connected to coaxial feed line 2 and the sleeve choke 12i act as a radiating element. The feed point of sleeve choke 12i is short-circuited and the other end thereof is open. The lengths of conductor 10 and sleeve choke 12i are designed so as to achieve maximum efficiency at a desired frequency.
The choke 12i is partially filled with dielectric material 16i that is dimensioned so that the choke forms a quarter wavelength transmission line and prevents coupling between the shell 14i and the extension 10 at the open end of the choke at the highest frequency. At some lower frequency of operation, the choke 12i becomes ineffective as an isolation element and the entire length P of the structure from the ground plane to the end of the conductor, becomes a monopole antenna at the lower resonant frequency.
The coupling between conductor 10 and sleeve choke 12i occurs at the open end of sleeve choke 12i. That is, when the length ##EQU1## the choke acts as a high impedance, whereby the coupling between conductor 10 and sleeve choke 12i is minimal. When ##EQU2## the choke acts as a low impedance, whereby the coupling between conductor 10 and choke 12i is higher. The electrical length of choke 12i can be adjusted by varying the dielectric constant of dielectric material 16i.
The construction consisting of internal and external conductors 10, 14i is regarded as coaxial transmission line, and its characteristic impedance is expressed as follows: ##EQU3## where .epsilon..sub.r is dielectric constant, D is the diameter of the external conductor, and d is the diameter of the internal conductor. The input impedance between internal and external conductors 10, 14i is denoted by the following equation: ##EQU4## where .gamma.=.alpha.+j.beta., .alpha. is attenuation factor, .beta. is propagation constant, l is length of transmission line, and Z.sub.L is load impedance.
In the antenna of FIG. 1, the ground plate 20 and external conductor 14i are structurally adjacent to each other, thereby causing parasitic capacitance which degrades the antenna efficiency. To improve the antenna efficiency, the parasitic capacitance can be decreased. Accordingly, in the construction of FIG. 1, the diameter of external conductor 14i must be reduced for this purpose, which is ultimately the same as the reduction of characteristic impedance of choke 12i according to the above equation (1). That is, such reduction in the characteristic impedance of choke 12i gives rise to a change in the amount of coupling, resulting in a degradation of the antenna's performance.
Thus, to minimally affect the amount of coupling and to keep the characteristic impedance of choke 12i essentially the same as it was previously (i.e., before the diameter of conductor 14i changed), the diameter of internal conductor 10 must be reduced. This results in a reduction in the antenna's bandwidth. Therefore, when the antenna is manufactured in such a manner, the same cannot satisfactorily cover the frequency bandwidth required for the system.
Further, since the dielectric material is employed to adjust the quantity of coupling, the dielectric constant and the dimension of the dielectric material must be accurately selected for proper coupling.